1. Field of the Invention
The present invention relates generally to a digital multimedia broadcasting system, and in particular, to a method and apparatus for supporting handover in a digital multimedia broadcasting system using a frame slicing technique.
2. Description of the Related Art
Currently, digital broadcasting is undergoing standardization based on various technologies by regions. For example, the standard broadcasting under discussion in China includes Digital Multimedia Broadcasting-Terrestrial (DMB-T), Advanced Digital Television Broadcasting-Terrestrial (ADTB-T), and Digital Video Broadcasting-Terrestrial (DVB-T).
DMB service is classified into Terrestrial DMB (DMB-T) and Satellite DMB (DMB-S) according to transmission media. In the world, Terrestrial DMB service is being deployed in Europe, and Satellite DMB takes initiative in the United States. Meanwhile, multimedia service including mobile TV service is expected to be deployed in the Far East region first in the world. A DMB-T transmission system is suitable for fixed terminals or portable/mobile terminals, but has a need for reduction in weight and power consumption to accommodate portable apparatuses and to reduce battery consumption.
Portable/mobile terminals have the following main requirements for the DMB-T system.
One requirement is power saving. Mobile portable terminals have lower required power consumption during radio frequency (RF) and baseband processing. However, in mobile portable terminals, average power consumption of supplemental receivers should be lower than this. This is because in the miniaturized environment, battery capacity is limited and heat dissipation is difficult. If new technology is introduced to mobile portable terminals in the future, a required decrement in power consumption can increase up to 90%.
Another requirement is smooth and seamless service handover. In a DMB-T Multi-Frequency Network (MFN), for mobile reception, there is a need to perform handover to another frequency if reception quality of the current frequency is too low. Because DMB-T does not include a seamless handover function, a change in the frequency causes service interruption. In addition, a receiver scans other available frequencies to find out which of them provides the best or sufficient reception quality. If the receiver does not include a separate RF stage for that purpose, the interruption occurs every time the receiver scans the frequencies. On the contrary, if the receiver includes a separate RF unit, the cost of the receiver increases. Therefore, there is a need to seamlessly perform handover and seamlessly scan a substitution frequency without the separate RF stage.
A further requirement is RF performance for mobile single antenna reception. A Carrier-to-Noise ratio (C/N) required for reception of RF signals is generally an important parameter that considerably affects the network cost, and especially affects the possibility of receiving a high-QoS (Quality of Service) service at a high reception rate.
A DVB transmission system mainly provides a bit rate of 10 Mbps or higher. Such a system introduces a Time Division Multiplexing (TDM)-based technique to make it possible to considerably reduce average power consumption of a DVB receiver. Such a technique is called a time slicing technique. A concept of the time slicing technique is to transmit data in bursts using a bit rate which is considerably higher than the bit rate required when data is continuously transmitted. A relative time until the start of the next burst is indicated by Δt (delta-t).
FIG. 1 is a diagram for a description of conventional time slicing. As illustrated, a terminal 100 is located in an overlapping area between a first cell (cell F1) 102 managing a first frequency F1, a second cell (cell F2) 104 managing a second frequency F2, and a third cell (cell F3) 106 managing a third frequency F3. Each of the cells 102 to 106 transmits the services available at its own frequency using the TDM technique, and repeats bursts of the same service at stated intervals. The services transmitted by each of the cells 102 to 106 may not be synchronized with each other.
For the terminal 100 receiving a service A from the cell F1 102, because data of an Element Stream (ES) is not transmitted between bursts of the service A, different ESs can use the bit rates allocated in different ways, as shown in FIG. 1. In this manner, the terminal 100 is activated only for the very short time where it receives bursts of the requested service. When the mobile portable terminal 100 requires a lower fixed bit rate, the required bit rate can be provided by buffering received bursts.
In order to obtain an appropriate power reduction effect, a burst bit rate should be at least 10 times the fixed bit rate of the provided service. For example, the bursts for a 350 Kbps streaming service should have a bit rate of about 4 Mbps. If the burst bit rate is 2 times the fixed bit rate, it can contribute to a power reduction of 50%, which does not reach the above-stated power reduction of 90%.
Power consumption differs according to a duty cycle of the time slicing technique. Estimation of power consumption takes into account not only the increase in power consumption due to Multi-Protocol Encapsulation—Forwarding Error Coding (MPE-FEC), but also the duty cycle. As a result, additional power consumption of 2 mW due to the use of a 0.13 μm technique and additional power consumption of 1 mW due to the use of a 0.18 μm technique for MPE-FEC are estimated.
It should be noted that such power consumption estimation is performed on the assumption that all Reed-Solomon (RS) codewords are always decoded. However, for a Moving Picture Experts Group-2 (MPEG-2) Transport Stream (TS), because it is already accurate and has no need for MPEG-FEC decoding, RS decoding is not used in a normal reception environment (especially in low low-rate reception environment) for the most time. Even though MPE-FEC is used, it is used only for the sub-set of the received burst. Therefore, in the complex reception environment (actual user environment), MPE-FEC consumes additional power of 2 mW on rare occasions, so an influence on battery time is not significant.
The time slicing technique can use a receiver to monitor adjacent cells for an off time. Switching between TSs for the off time does not cause interruption of the service reception. If bursts of a particular IP stream are synchronized between the adjacent cells through an appropriate action, the receiver can continuously receive the IP stream without a data loss when it is tuned to the adjacent cells. The time slicing technique aims at a reduction in power consumption at a mobile portable terminal.
Therefore, time slicing should be optimized from the viewpoint of a terminal. Such selection follows a DVB adoption rule that implementation in a receiver should be optimized because the number of receivers is much greater than the number of transmitters. In addition, commonly the implementation cost at a network is less important than the implementation cost at a terminal. The time slicing supports to enable receivers to monitor adjacent cells during off time. Performing switching between transport streams for an off period does not lead to interruption of service reception.
As illustrated in FIG. 1, the terminal 100 receives the service A from the F1 102, but the quality of the received service is low. Then the terminal 100 listens to (or searches for) other frequencies, i.e. F2 and F3, in off times 112 and 114 between bursts 110 of the service A. The terminal 100 listens to the F2 in the first off time 110, and the F3 in the second off time 114. The terminal 100 listens to the cell F2 104 and the cell F3 106 in the first and second off times 112 and 114, respectively, and compares the listening results to make a switch to the best cell.
FIG. 2 shows conventional handover by time slicing. A terminal receives a service A of one burst from a cell F1 in step 202. The cell F1 is a serving cell of the terminal. If a first off time starts as the burst of the service A terminates, the terminal listens to the signal quality to determine whether the service A exists in a cell F2 in step 204. In FIG. 1, the terminal listens to services B, C, D, E and F in the first off time 112 for the cell F2. Upon success in listening to the service A, the terminal stores, in step 206, the corresponding information, returns to the cell F1, and then turns the power off until the next burst of the service A starts. In step 208, the receiver receives the burst of the service A.
If a second off time starts as the burst of the service A terminates, the terminal listens to the signal quality to determine whether the service A exists in a cell F3 in step 210. In FIG. 1, the terminal listens to services D, E, F and A in the second off time 114 for the cell F3. Because the terminal has succeeded in listening to the service A, it stores, in step 212, the corresponding information, returns to the cell F1, and then turns the power off until the next burst of the service A starts. In step 214, the receiver receives bursts in the allocated time for the service A. If frequency listening for all adjacent cells except for the cell F1 is completed between the bursts of the service A, the terminal compares, in step 216, signal qualities for the adjacent cells from which it will receive the service A, to select the best cell, and then switches to the selected best cell to receive the service A.
The foregoing conventional time slicing technique has at least the following problems.
First, when listening to other cells, the terminal needs to be powered off until it listens to a desired service.
Second, a position of the service burst affects the listening result in the time slicing. Referring to FIG. 1, the reception quality of the service A deteriorates in the F1. During the first off time, the terminal listens to the F2. However, because a position of the service A in the F2 is equal to that in the F1, the terminal cannot find the service A, and thus returns to the F1. As a result, even though the service quality in the F2 is highest, the terminal cannot find the F2. In order to enable adjacent cells to transmit the same service through several different time slices at the same time, there is a burden that it should perform careful synchronization at a head end of the service.